This invention pertains to a fluid operated control system for a variable displacement pump or pumps driven by a prime mover such as an internal combustion engine, as in a materials handling vehicle such as an earth mover. The invention is more specifically directed to a system for controlling the per cycle displacement of the pump or pumps to suit the torque requirement thereof to each of several different output conditions of the prime mover.
Typical of known pump displacement control systems of the type under consideration is the one wherein the torque demand (the product of the delivery rate and pressure) of a variable displacement pump is predetermined to correspond to the output torque rating of the internal combustion engine driving the pump. The pump displacement is controlled so as to maintain its torque requirement at the predetermined value regardless of the different output conditions into which the engine may be set by the control lever of the fuel injection pump. The constant torque control of the pump is subject to the objection that the torque rise of the engine when it is in less than the maximum output condition cannot possibly be utilized for driving the pump.
An additional drawback of the known constant torque pump control system also manifests itself when the engine is working in such a speed setting that the maximum engine output torque falls short of the torque rating. The engine has been easy to stall when the horsepower demand of the pump exceeds the maximum horsepower capability of the engine in that output condition. The fuel consumption efficiency of the engine has also decreased by reason of its overloading.